The expression “laser cutting process” is intended to refer, for the purposes of the present invention, to a process in which a laser beam focussed on the surface of a workpiece, or near that surface, produces a transformation of the material of the workpiece hit by the laser beam to obtain first a through hole and then a cut line starting from this through hole. The relative movement of the laser beam with respect to the workpiece determines the overall area, or volume, of material involved by the process. Typically, the transformation of the material due to the process is either a transformation of mechanical type (deformation) or a transformation of physical type (phase transition by fusion, evaporation or sublimation) and is due to the following two main factors, combined in variable proportions:
a) the heat supplied by the focussed laser beam; and
b) the heat supplied by a chemical reaction caused by a so-called assisting gas, provided such a reaction is an exoenergetic one (typically a reaction of combustion, or more generally a reaction involving the combination of the assisting gas with the material of the workpiece).
In case the heat supply indicated above with b) must not be provided for, the assisting gas is an inert gas (such as for instance N2, Ar or He) and has the function of shielding or of mechanical propulsion (i.e. it serves to blow away the material which has fused, evaporated or sublimated as a result of the heat supplied by the laser beam).
On the contrary, in case the heat supply indicated above with b) must be equal to or larger than 40% of the total energy supply, the assisting gas is a reactive gas and acts as energy-yielding means or as comburent. The role of the assisting gas in the laser working process is therefore in this case to yield energy to the process by means of an exoenergetic reaction, with two simultaneous effects on the process: 1) increase in the temperature of the volume of material involved, which results in a physical change of state due to thermal effect (plasticization, fusion, evaporation or sublimation); and 2) self-sustainment of the reaction, in that the temperature of the volume of material involved and the available heat energy ensure the conditions required to cause and sustain the exoenergetic reaction. An example of assisting gas of reactive type is oxygen (O2), which is used in laser working operations performed on carbon steel alloys, since it is able to sustain a reaction of oxidization of the iron contained in the steel.
Laser piercing as a preliminary phase of cutting is usually carried out with no relative movement of the laser beam with respect to the workpiece and is aimed at causing breaking of the wall of material in view of the subsequent cutting process. Laser piercing is carried out with an optical configuration and with a position of the focal point relative to the material which must be also compatible with the cutting process which takes place immediately after the wall of material has been broken. Laser piercing takes place in a volume which remains closed until the end of the process. As schematically illustrated in FIG. 1 of the attached drawings, the laser piercing process involves first the surface S of the workpiece P, then evolves creating a cylinder which comprises, starting from the optical axis A of the laser beam, a space which collects evaporated/sublimated, fused and heated material, in an atmosphere which comprises the assisting gas, possible by-products deriving from chemical reactions between the material of the workpiece and the copresent gases, as well as possible other gases contained in the air in which the workpiece being processed is placed, which gases are present as contaminants.
Differently from piercing, the laser cutting process provides for a relative movement of the focussed laser beam with respect to the workpiece. Moreover, as schematically shown in FIG. 2 of the attached drawings, the laser cutting process takes place in an open volume defined by three surfaces, namely by a pair of flat surfaces S1, S2 which extend parallel to the direction of the relative movement of the focussed laser beam with respect to the workpiece, and by a third surface S3 which connects the first two surfaces and represents the leading edge of the cut. As schematically shown in FIG. 3 of the attached drawings, which is a section view of a wall of material being cut by means of laser, which view is taken through a section plane parallel to the direction of the cut, the leading edge of the cut is formed by various layers of heated, fused and evaporated/sublimated material, in an atmosphere which comprises the assisting gas, possible by-products deriving from chemical reactions between the material of the workpiece being processed and the copresent gases, as well as possible other gases contained in the air in which the workpiece is placed, which gases are present as contaminants.
Document U.S. Pat. No. 5,373,135 discloses a method for controlling a laser cutting process based on setting two temperature thresholds, namely a minimum temperature threshold and a maximum temperature threshold, respectively, corresponding to the fusion temperature of the material being processed and to a temperature comprised between the fusion temperature and the evaporation temperature of the material being processed, and on measuring the temperature by measuring the light intensity. When the measured temperature is higher than the predetermined maximum threshold, then the laser is switched off, whereas when the measured temperature is lower than the predetermined minimum threshold, then the laser is switched on. The control parameter of this known method is therefore the temperature.